Quick Revision for Class X Physics SA1

  1. Ohm: It is the S.I. unit of resistance. A conductor has a resistance of one ohm if a current of one ampere flows through it on applying a potential difference of one volt across its ends.                                                                                                                                1 ohm = 1 volt/1 ampere        or      1Ω = 1V/1A
  2. Factors on which resistance of a conductor depends:  The resistance R of a conductor depends                                                                                               

i) Directly on its length L i.e. R α L.                                                                                                                                                                         ii) inversely on its area of cross-section A i.e. R α 1/A                                                                                                                                                       iii) on the nature of material of the conductor on.                                                                                                                                                                     On combining the above factors, we get                                                                                                                                                                                                 R α L/A                                                                                                                                                                                                                                                                        R = ρ * L/A   The proportionality constant ρ is called
resistivity of conductor.

  1. Resistivity: It is defined as the resistance offered by a cube of a material of side 1 m when current flows perpendicular to its opposite faces. Its S.I. unit is ohm-meter (Ωm).                                                                                                                                                                    Resistivity, ρ = RA/L
  2. Equivalent resistance: If a single resistance can replace the combination of resistances in such a manner that the current in the circuit remains unchanged, then that single resistance is called the equivalent resistance.
  3. Laws of resistances in series:                                                                                                                                                                                       i) Current through each resistance is same.                                                                                                                                                                         ii) Total voltage across the combination = Sum of the voltage drops.                                                                                                                 V= V1 + V2 + V3                                                                                                                                                                                                                         iii) Voltage drops across any resistor is proportional to its resistance.                                                                                                                       V1 = IR1, V2 = IR2, V3 = IR3                                                                                                                                                                                                             iv) Equivalent resistance = Sum of the individual resistances.                                                                                                                                                 Rs = R1 + R2 + R3                                                                                                                                                                                                                              v) Equivalent resistance is larger than the largest individual resistance.
  4. Laws of resistances in parallel:                                                                                                                                                                                                              i) Voltage across each resistance is same and is equal to the applied voltage.                                                                                                                  ii) Total current = Sum of the currents through the individual resistances.     I = I1 + I2 + I3                                                                                                                                                                                                                                                   iii) Currents through various resistances are inversely proportional to the individual resistances.                                                                             I1 = V/R1, I2 = V/R2, I3 = V/R3                                                                                                                                                                                                     iv) Reciprocal of equivalent resistance = Sum of reciprocals of individual resistances.                                                                                                1/Rp = 1/R1 + 1/R2 + 1/R3                                                                                                                                                                                                          v) Equivalent resistance is less than the smallest individual resistance.
  5. Joule’s law of heating: It states that the heat produced in a conductor is directly proportional to (i) the square of the current I through it (ii) proportional to its resistances R and                                                                                                     (iii) the time t for which current is passed. Mathematically, it can be expressed as                                                                                                                                                                                                               H = I2Rt           joule = I2Rt/4.18 cal                                                                                                                                                                                                or                                                                                                                                                                                                                                                     H = VIt             joule = VIt/4.18cal
  6. Electric energy: It is the total work done in maintaining an electric current in an electric circuit for given time.                                                                      

Electric energy, W = VIt = I2Rt joule

  1. Electrical power: Electrical power is the rate at which electric energy is consumed by an appliance.

P = W/t = VI = I2R = V2/R

  1. Watt: It is the S.I. unit of power. The power of an appliance is 1 watt if one ampere of current flows through it on applying a potential differences of 1 volt across its ends.                                                                                                                                      1 watt = 1 joule/1 second =1 volt x 1 ampere                                                                                                                                                      or  1 W = 1 Js-1 = 1 VA                                                                                                                                                                                        1 kilowatt = 1000 W          1Mega watt= 106 watt                      1Gigawatt=109 watt                     


  1. Kilowatt hour: It is the commercial unit of electrical energy. One kilowatt hour is the electric energy consumed by an appliance of 1000 watts when used for one hour.                                                                                                                                                              1 kilowatt hour  (kWh) = 3.6 x 106 J

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